More than one person can potentially sign up for the same shift despite the maximum limit

Scheduled shifts were configured correctly for Maximum Staff and Waitlist behavior as Blocked and normally this would prevent overflow signups.

After further investigation it was found that these are the result of situations where signups usually happen close together. The timing of when the system checks if the Shift is full is when the volunteer first visits the shift selection page, but it doesn't check it again once they actually make their shift selection. This presents a window in where more than one person can potentially sign up for the same shift despite the maximum limit.